Acid recovery process



Patented Dec. 4, 1945 Henry 0. Mottern, Hillside, N. J., assignor to Standard Oil Development Company, a corporation of Delaware No Drawing. Application October 7,-194 SerialNo. 481,230

3 Claims. (01. 23-172) The present invention relates to a process for reconditioning and recovering spent .acidsj and, more particularly, the invention involves a process for recovering spent sulfuric acid which has been used in an olefin hydration process.

Olefin hydration processes, in brief, consist in the absorption of olefin in 60-98% sulfuric acid, which forms an extract of alkyl acid esters. The concentrated extract is then diluted with water to an acid concentration of 35-50% acid strength on an olefin-free basis, hydrolyzed and steam stripped to recover olefin hydration products. The residue from the distillation step is a weak acid, commonly designated as slop acid, and is drained from the still. The weak acid contains from 1 to 5% of carbon in several forms, that is alcohol, ketone, sulfonic acid, hydroxy, and polyhydroxy sulfonic acid, resin and tar. Restoration of such acid without specific decarbonization treatment yields a strong acid which is not suitable for re-usefcr olefin absorption because of its high carbon content, which would precipitate as elemental carbon upon dilution of the strong acid after further olefin absorption. Where sulfuric acid is re-used for olefin absorption it is most desirable to use a reclaimed acid containing not more than 0.2% carbon and preferably the concentration of carbon should be 0.1% or less.

Present practice in the reconditioning of weak acid calls for concentrating the acid to an 88-98% acid strength, and'then decarboniz'ation' by either bleaching the acid by chemical agents or by the prolonged digestion of the acid. restored to an acid concentration of from 88-98% acid strength by evaporation or fortification with S0: which causes considerable loss of sulfuric acid as S02.

Restoration of the weak acid through bleaching is accomp i d by concentrating the weak acid to about 88% acid concentration, then adding' an excess of nitric acid and heating to 210 C.

to oxidize the carbonaceous material present in the acid. The amount of. nitric acid required by this method is not uniform, and it often requires twice as much nitric acid to obtain complete oxidation of the carbonaceous material as that theoretically required judging from a total carbon content of the weak acid. When the mixture of sulfuric and nitric acids is heated as the final bleaching step all of the nitric acid present is lost either as nitrogen or nitrogen oxides. Thi

process results in a good recycle acid, but the cost in nitric acid is excessive;

The principal object of the present invention is to accomplish the bleaching of recycled sulfuric complished without the loss of substantial quantities of nitric acid. This and other objects will be apparent to those skilled in the art upon reading the following description and claims.

According to the present invention the bleachihg'of recycle acid is accomplished by adding the.

bleaching agent, in an amount only slightly in excessover that theoretically required to oxidize the carbonaceous material present in the weak acid, to the weak acid as it leaves the alcohol still. The mixture of weak acid and nitric acid is then heated. At temperatures up to 105 C. a preliminary oxidation occurs-- yielding NO and N02. As the temperature rises, a mixture of water and nitric acid is distilled oil from which a constant boiling mixture of' nitric acid and water is recovered by fractional distillation. At

146 C. the weak acid has been concentrated to' an acid concentration of approximately on an olefin-free basis. Heating is then continued and the acid mixture further concentrated to an acid concentration of on an olefin-free basis which is reached at a temperature of approximately 205 C; -As the acid concentration is raised-from 65% to that of approximately 80%.

various nitrogen oxides are liberated along with some water vapors. These nitrogen .oxides to-' gether with any water vapors present are then conducted to an absorption tower where they are contacted with water or weak sulfuric acid from the olefin hydration process in cou'ntercurrent-flow. They are here absorbed and converted to nitric acid according to the following equationsz -2NO +O2-2NO2 2NOa-l-HzO HNO:+I-INOr A small amount of air or free oxygen should be fed into the absorption tower along with the oxides of nitrogen to furnish the oxygen requisite for the conversion of the oxides to nitric acid. The nitrogen oxides liberated during the preliminary oxidation are converted to nitric acid in the I same manner. After a sulfuric acid concentration of 80% has been reached and all of the remaining nitric acid has been liberated as oxides of nitrogen, the sulfuric acidis then in a condition where it can'be further concentrated up to 88-92% acid concentration. The sulfuric acid at this concentration can then be used, without further treatment, for recycling to an olefin absorption system. The process can be run either continuous or on the batch principle with very little acid with nitric acid, such bleaching being ac- 55 loss of nitric acid.

As has been previously stated, the weak acid tion process has aesulfuric acid content of'from 35 to 55% strength on an olefin-free basis. and con ains from 1- of carbonaceousmaterial.

- Concentrated nitric acid isthen added to the weak acid in an amount equal to 5 to 25% based on the H2804 present in the weak acid figured, as 100%, where the organic material content in the weak acid is from 1 to 5%. Dilute nitric acid,

recycle nitric acid or nitrogen oxides may be used instead of concentrated nitric acid if care is taken to maintain the smog in slight excess, over that required to oxidize the carbonaceous material.-

The weak acid is drained from the steam distillatipn still at a temperature within a few degrees of 100 C. and is run along. with the added nitric acid into an externally heated still equipped with drained from' the steam still in an olefin hydraresulting in the production of additional NOand' N02. These oxides were also, absorbed in the NaOH solution and the total quantity of NO and NO, was determined. by titration. 14.75 g'.- of HNOrwere recovered as NO and N02. The total HNO: recovered was 16.85 g. or 98.2% of the nitric acid charged. :Distillation was continued until the sulfuric acidconteut reached 92%. 325 g. of

92% sulfuric acid havinga carbon content of .028% were recovered.

What is claimed is: a

1. The method of reconditioning weak solutions of sulfuric acid containing carbonaceous material which. comprises adding nitric acid to' i the weak acid, heating the acid mixture while oxides .of nitrogen are evolved, concentrating the acid to a sulfuric acid concentration of 80-81% a fractionating tower. The acids are thus heat d v to cause the preliminary oxidation andv are then concentrated as previously described, and an amount of a constant boiling mixture of nitric add and water (B.'P. row-110 c.) isrecovered constant boilingnitric acid mixture to fresh weak acid-feed at least 98% of the original nitric acid charged is conserved.

' The following specific example is given for the purpose of illustrating the principles'of this in-] vention.

- Esamplc A sulfuric acid, recovered from the secondary butyl alcohol steam stripping still, containing 1.10% carbon as carbonaceous material, was mixed with concentrated nitric acid in the following proportions:

- with a fractionating column and heated. As the temperature passed through the range of 80-105 0. oxidation became rapid and NO and NO; gases were given 011'. For purposes of calculation these oxides were absorbed in NaOH solution. Distillation was continued to concentrate the acid. Between 105-110 C. a distillate containing 2.1 g. or .HNO: was recovered. After suillcient water hacl'been removed to bring the sulfuric acid strength to 80-81% a further oxidation took place furic acid concentration of 80-8 while recovering a constant boiling mixtureoi' nitric acid and water, further heating the mixture to evolve oxides of nitrogen and after oxides of nitrogen cease to. evolve concentrating the Iacid to a sulfuric acid concentration of 88-92%.

2. The method of reconditioning weak solutions of sulfuric acid containing carbonaceous material. which comprises adding to the weak facid an amount of nitric acid slightly in excess of the amount theoretically required to oxidize the carbonaceous material present in the weak acid,

heating the acid mixture while oxides of nitrogen are evolved, concentrating the acid to a sul- 1% while recoveri'rig a constant boiling mixture of nitric acid and water, further heating the mixture to evolve oxides of nitrogen, returning the nitric acid and nitrogen tetraoxide to fresh weak acid and after oxides of nitrogen cease to evolve concentrating the sulfuric acid to "an 88-92%.

'3. The method of retonditibning'weak soluacid concentration of acid an amount of nitric acid slightly in excess of the amount theoretically required tooxidize the carbonaceous material present in the weak acid, heating the acid mixture to a temperature of 105 C, while oxides of nitrogen are evolved, which oxides are returned to fresh weak solutions of sulfuric acid, concentrating the acid to a sulfuric acid concentration of -81% whilerecovering a' mixture of nitric acid and water boiling between -110 C. which nitric acid and water mixture is returned to fresh weak acid, further heating the mixture to evolve oxides of nitrogen which are returned to fresh weak acid as nitrogen tetraoxide and after oxides of nitrogen cease to evolve concentrating the acid toa sulfuricacid concentration of 88-92%.

" max 0. iso'rraan.

tions of. sulfuric acid containing carbonaceous material which eompries adding to the weak I 

